Fuel cells are electrochemical devices in which part of the energy of a chemical reaction is converted directly into direct current electrical energy. The direct conversion of energy into direct current electrical energy eliminates the necessity of converting energy into heat thereby avoiding the Carnot-cycle efficiency limitation of conventional methods of generating electricity. Thus, without the limitation of the Carnot-cycle, fuel cell technology offers the potential for fuel efficiencies two to three times higher than those of traditional power generator devices, e.g., internal combustion engines. Other advantages of fuel cells are quietness, cleanliness (lack of air pollution) and the reduction or the complete elimination of moving parts.
Typically, fuel cells contain two porous electrical terminals called electrodes with an electrolyte disposed therebetween. In the operation of a typical fuel cell, an oxidant is continuously introduced at the oxidant electrode (cathode) where it contacts the electrode and forms ions thereby imparting positive charges to the cathode. Simultaneously, a reductant is continuously introduced at the fuel electrode (anode) where it forms ions and leaves the anode negatively charged. The ions formed at the respective electrodes migrate in the electrolyte and unite while the electrical charges imparted to the electrode are utilized as electrical energy by connecting an external circuit across the electrodes. Fuel cell reactants are classified as oxidants and reductants based on their electron acceptor or electron donor characteristics. Oxidants include pure oxygen, oxygen-containing gases (e.g., air) and halogens (e.g., chlorine). Reductants include hydrogen, carbon monoxide, natural gas, methane, ethane, formaldehyde and methanol.
The electrolyte of the fuel cell serves as the electrochemical connection between the electrodes providing a path for ionic current in the circuit while the electrodes, made of carbon or metal, provide an electrical pathway. Further, the electrolyte prevents transfer of the reactants away from the respective electrodes where the formation of explosive mixtures can occur. The electrolyte utilized must not react directly to any appreciable extent with the reactants or reaction products formed during the operation of the fuel cell. Further, the electrolyte must permit the migration of ions formed during operation of the fuel cell. Examples of electrolytes that have been used are aqueous solutions of strong bases, such as alkali metal hydroxides, aqueous solutions of acids, such as sulfuric acid and hydrochloric acid, aqueous salt electrolytes, such as sea water, fused salt electrolytes and ion-exchange polymer membranes.
One type of fuel cell is a polymer electrolyte (PEM) fuel cell which is based on a proton exchange polymer membrane. The PEM fuel cell contains a solid polymer membrane, which is an "ion-exchange membrane" that acts as an electrolyte. The ion-exchange membrane is sandwiched between two "gas diffusion" electrodes, an anode and a cathode, each commonly containing a metal catalyst supported by an electrically conductive material. The gas diffusion electrodes are exposed to the respective reactant gases, the reductant gas and the oxidant gas. An electrochemical reaction occurs at each of the two junctions (three phase boundaries) where one of the electrodes, electrolyte polymer membrane and reactant gas interface.
For example, when oxygen is the oxidant gas and hydrogen is the reductant gas, the anode is supplied with hydrogen and the cathode with oxygen. The overall chemical reaction in this process is: 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O. The electrochemical reactions that occur at the metal catalyst sites of the electrodes are as follows:
anode reaction: 2H.sub.2 .fwdarw.4H.sup.+ +4e.sup.- PA1 cathode reaction: O.sub. +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O
During fuel cell operation, hydrogen permeates through the anode and interacts with the metal catalyst, producing electrons and protons. The electrons are conducted via an electronic route through the electrically conductive material and the external circuit to the cathode, while the protons are simultaneously transferred via an ionic route through the polymer electrolyte membrane to the cathode. Concurrently, oxygen permeates to the catalyst sites of the cathode, where the oxygen gains electrons and reacts with the protons to yield water. Consequently, the products of the PEM fuel cell reactions are water and electricity. In the PEM fuel cell, current is conducted simultaneously through ionic and electronic routes. Efficiency of the PEM fuel cell is largely dependent on the ability to minimize both ionic and electronic resistivity to current.
Ion-exchange membranes play a vital role in PEM fuel cells. Improved membranes have substantially increased power density. In PEM fuel cells, the ion-exchange membrane has two functions: (1) it acts as the electrolyte that provides ionic communication between the anode and cathode; and (2) it serves as a separator for the two reactant gases (e.g., O.sub.2 and H.sub.2).
Optimized proton and water transports of the membrane and proper water management are crucial for efficient fuel cell application. Dehydration of the membrane reduces proton conductivity, and excess water can lead to swelling of the membranes and flooding of the electrodes. Both conditions result in poor cell performance. In the fuel cell, the ion-exchange membrane, while serving as a good proton transfer membrane, also must have low permeability for the reactant gases to avoid crossover phenomena that reduce performance of the fuel cell. This is especially important in fuel cell applications in which the reactant gases are under pressure and the fuel cell is operated at elevated temperatures. Therefore, a good ion-exchange membrane for a PEM fuel cell has to meet the following criteria: (1) chemical and electrochemical stability in the fuel cell operating environment; (2) mechanical strength and stability under cell operating conditions; (3) high proton conductivity, low permeability to reactant gas, and high water transport; and (4) low production costs.
An electrolyte double-layer capacitor comprises a separator layer positioned between polarizable electrodes. Almost every double-layer capacitor uses either filter paper or thin prose polypropylene film. Polypropylene (PP) is very hydrophobic with high electric resistance, which results in an increased internal resistance of the double-layer capacitor. Therefore, a good ion-exchange membrane for an electrolyte double-layer capacitor has to meet the following criteria: (1) chemical and electrochemical stability in the operating environment; (2) mechanical strength and stability under operating conditions; (3) high ion conductivity; (4) low electric resistance, and (5) low production costs.
Zinc-halogen rechargeable cells have a high theoretical energy density. In those cells, an aqueous zinc halide salt electrolyzes to zinc metal, which deposits on an anode, while molecular halides accumulate at a cathode. As the cell discharges, zinc and molecular halide react back into the salt form. However, diffusion of the halides from the cathode to the anode causes internal self-discharge of the cell when molecular halides react with zinc deposited on the anode. Therefore, a separator must be placed between the two electrodes. However, these membranes have a high internal resistance, and are very expensive. Therefore, a good ion-exchange membrane for a zinc-halogen rechargeable cell has to meet the following criteria: (1) chemical and electrochemical stability in the operating environment; (2) mechanical strength and stability under cell operating conditions; (3) electric resistance that can be adjusted, and (4) low production costs.
A variety of membranes have been developed over the years for electrochemical applications, such as solid polymer electrolytes in fuel cells, electrolyte double-layer capacitors, and rechargeable zinc-halide cells. Sulfonic acids of polydivinylbenzene-styrene based copolymers have been used as electrolytes in fuel cells. Perfluorinated sulfonic acid membranes developed by DuPont and Dow Chemical Company also have been used as electrolytes in fuel cells. DuPont's Naflon.RTM. membrane is described in U.S. Pat. Nos. 3,282,875 and 4,330,654. Nafion.RTM.-type membranes have high stability and good performance in fuel cell operations. However, they are relatively expensive to produce.
Alternatively, a series of low cost, ion-exchange membranes for PEM fuel cells have been investigated. U.S. Pat. No. 5,422,411 describes trifluorostyrene copolymers that have shown promising performance data as membranes in PEM fuel cells. Sulfonated poly(aryl ether ketones) developed by Hoechst AG are described in European Patent No. 574,891, A2. These polymers can be crosslinked by primary and secondary amines. When used as membranes and tested in PEM fuel cells, these polymers exhibited only modest cell performance. A series of low cost, sulfonated polyaromatic based systems, such as those described in U.S. Pat. Nos. 3,528,858 and 3,226,361, also have been investigated as membrane materials for PEM fuel cells. These materials suffer from poor chemical resistance and mechanical properties that limit their use in PEM fuel cell applications.
Polymer blending is a simple, more feasible technology than methods that compound different polymer segments via copolymerization or the formation of inter-penetrating materials. Homogeneous polymer blends consist of two polymers that are miscible at the molecular level and combine the properties of the components to yield a distinct new material. However, very rarely does the blending of polymers result in a homogenous polymer blend because in general, polymers do not mix homogeneously, even when they are prepared using the same solvent.
In most cases, Gibbs' free energy of mixing [.DELTA.G=.DELTA.H-T.DELTA.S] of polymers is a positive value because the entropy of mixing (.DELTA.S) of high molecular macromolecules approaches zero when the molecular weight of the polymers is greater than 10,000. Unless the enthalpy of mixing (.DELTA.H) is negative or at least equal to zero, polymers are not miscible and attempts to blend the polymers results in phase separation in the "blend" resulting in poor mechanical strength, i.e., a non-homogenous "blend" that retains the distinct phases of the pure polymers and in most cases, poor interaction between the phases occurs. Thus, the non-homogenous "blend" falls apart or has a much weaker structure than the original polymers.
Miscibility of polymers occurs in their amorphous regions. If one polymer in a two polymer blend is a semi-crystalline material, the crystal structure of the polymer retains its purity in the blend. However, its melting point usually decreases when the two polymers in the blend are miscible. Therefore, if two polymers are miscible, and one of the polymers is semi-crystalline, a semi-crystalline polymer blend is formed in which the amorphous structure is miscible. The different amorphous phases of the two polymers do not separate, but the crystalline component spreads within the amorphous structure and serves as "crosslink" junctures.
The crosslinking term when applied to crystalline junctures does not refer to chemical crosslinking as in chemical or radiation treatment. Rather in this context, it refers to what occurs because the crystals are composed of macromolecules that extend into the amorphous structure and, thus interact and blend with the polymer chains of the non-crystalline polymers. Therefore, the crystalline structure is tied up to the amorphous structure in polymer blending by polymer molecules that partially take part in the building of the crystal and are partially amorphous. These polymer molecules take part in the amorphous form and interact with other miscible polymers. For example, it is expected that a polymer blend, semi-crystalline film will exhibit a much higher tensile strength than the theoretical arrhythmic weight average of the pure polymer component. Also, it is expected that miscible polymers in a blend will display homogeneity with regard to some desired properties such as optical clarity, glass transition temperatures and for membrane purposes, improved mass transport properties.
Considerable research has been done in attempts to prepare blend polymer membranes. However, only a few membrane systems have been discovered. Y. Maeda et al., Polymer, 26, 2055 (1985) report the preparation of blend membranes of poly(dimethylphenylene oxide)-polystyrene for gas permeation. They found this system to exhibit permeation rates unlike the permeation rates of either of the blend's polymer components.
Poly(vinylidene fluoride), PVF.sub.2, is a hydrophobic polymer that is used as a membrane in microfiltration and ultrafiltration. Bernstein et al., Macromolecules, 10, 681 (1977) report that a blend of PVF.sub.2 with poly(vinyl acetate) increases hydrophilicity of such hydrophobic membrane, which is needed in order to ultrafiltrate aqueous solutions. They found the macromolecules of the two polymers to be miscible at the molecular level. However, very few scientific tools are provided to predict a blend polymer membrane suitable for use in electrochemical cells.
In our prior U.S. patent application Ser. No. 08/725,747, published as WO 97/24777 on Jul. 10, 1997, the disclosure of which is incorporated herein by reference, we disclosed and claimed, inter alia, a polymer electrolyte membrane containing an ion-exchange polymer membrane, the ion-exchange membrane comprising a blend of a homogeneously sulfonated poly(phenylene oxide) and poly(vinylidene fluoride), wherein the homogeneously sulfonated poly(phenylene oxide) had a number average molecular weight between about 15,000 and about 10,000,000 and an ion charge density between about 1 and about 3.9 meq/g, wherein the poly(vinylidene fluoride) has a number average molecular weight between about 10,000 and about 10,000,000, and wherein the weight ratio of homogeneously sulfonated poly(phenylene oxide) to poly(vinylidene fluoride) in the blend was between about 1 to 1 and about 20 to 1.
It is, therefore, an objective of the invention to produce a low cost, easy to prepare ion-exchange polymer membrane with favorable chemical and mechanical properties for PEM fuel cell and other electrochemical applications.
Another object of the invention is to provide an improved solid polymer electrolyte fuel cell having a high current density, e.g., between 1 A/cm.sup.2 and 2 A/cm.sup.2 at 0.5 V, using a very low loading electrode equivalent to a platinum loading of between 0.1 and 0.2 mg/cm.sup.2 on a platinum/carbon/PTFE electrode at 30 psi reactant gases.
Another object of the invention is to provide an improved electrical double-layer capacitor having a specific capacitance, e.g. between 20 farad/cm.sup.3 and 60 farad/cm.sup.3.
Another object of the invention is to provide a rechargeable zinc-halide cell having a high current and voltage efficiency, e.g., between 80% and 100% when tested for 800 cycles (3 h/3 h), and a high energy density, e.g., between 70 mWh/cm.sup.3 and 100 mWh/cm.sup.3.
Another object is to provide novel homogeneous blends of sulfonated poly(phenylene oxide) with poly(vinylidene fluoride).